Site 1192 penetrated 355.5 m of sediment through seismic Megasequences D, C, and part of B (Fig. F29). The site is located on multichannel seismic line MAR20 at shotpoint 4126, a location that lies midway between the NMP and the Southern Marion Platform (SMP) on top of an ~50-m-deep incision in the basement surface (Fig. F7 in the "Leg 194 Summary" chapter; F29).
Because the primary objective of Site 1192 was an engineering test for the HYACE tools, no downhole logging operations were performed. Consequently, no velocity log or check shot information is available to tie the seismic record with maximum accuracy to the cores. The time-to-depth conversion is calculated by integrating shipboard velocity measurements performed with the PWS tool (see "Core Physical Properties"; the reliability of the PWS velocity data is discussed within that section). The resulting traveltime-to-depth conversion for seismic reflections and sequence boundaries is shown in Figure F30. A synthetic seismogram was calculated with the GRA bulk density and the PWS velocities. It is superimposed on line MAR20 and displayed at the left on Figure F30. A normal polarity zero-phase wavelet of 80 ms in length was statistically extracted from the seismic data of line MAR20. No time-control points were used for calculation of these synthetic seismograms.
The seismic facies of Megasequence D is characterized by laterally continuous reflections that dip gently toward the southeast. Reflection amplitudes are generally low, with one prominent intramegasequence high-amplitude reflection at 650 ms two-way traveltime (TWT). The southeast dip of the reflections seen on line MAR20 (Fig. F7 in the "Leg 194 Summary" chapter) results in an eastward thickening of Megasequence D. This thickening forms a drift wedge toward the preexisting topographic high of the SMP. Low-angle lapout and toplap reflections within the wedge indicate that modern sedimentation at Site 1192 is condensed or that the seafloor is an unconformity surface. In addition, several smaller-scale unconformities within Megasequence D indicate the occurrence of hiatuses within the drift package. The seismic Megasequence C/D boundary is defined along line MAR20 toward the southeast by a downlap onto underlying seismic Megasequence C at the base of the SMP escarpment and toward the northwest by an onlap onto the NMP (Fig. F7 in the "Leg 194 Summary" chapter).
Seismic Megasequence D incorporates lithologic Unit I and part of Unit II, ranging in age from late Miocene to Pleistocene. The absence of Holocene sediments at this site confirms the assumption that modern sedimentation is strongly reduced or absent and that the actual seafloor represents an unconformity. The high-amplitude intrasequence reflection at ~650 ms TWT, marking a seismic sequence boundary within Megasequence D, coincides with a downcore increase in bulk density at 81 mbsf (upper dashed line in Fig. F30). The interpretation of Megasequence D to be a drift deposit is based on seismic geometry and is supported by the increased siliciclastic fraction in lithologic Subunit IIA (2.5-103 mbsf). In addition, the dominant wackestone to packstone lithology of this interval also contrasts with the underlying dominant mudstone of Subunit IIB (see "Lithostratigraphy and Sedimentology"). Megasequence C/D boundary, positioned at ~120 mbsf, is characterized southeast of Site 1192 by a change in shedding direction, because the dip of the reflections changes at the base of the SMP upcore from northwest to southeast (Fig. F8 in the "Leg 194 Summary" chapter). This change indicates a halt to the export of platform-derived material from the SMP to the northwest. Such a change in sedimentation pattern could be caused by an exposure and/or drowning of the SMP around or just prior to the Miocene/Pliocene boundary, which would reduce the carbonate input into the current-controlled depositional system. In the area of Site 1192, this sequence boundary is conformable, and no large stratigraphic hiatus was observed in the drill cores. The applied time-depth correlation places the seismic Megasequence C/D boundary at 120 mbsf (Fig. F30), which coincides with an age of ~7.2 Ma (see "Age Model").
At this site, the overall seismic facies character does not change across seismic Megasequence C/D boundary. The amplitudes of the laterally continuous reflections remain low, and only one couplet of reflections displays a high amplitude at 750 ms TWT (Fig. F29). Regionally, Megasequence C wedges out toward the northwest by forming an onlap onto underlying Megasequence B/C boundary, as can be seen on line MAR20 (Fig. F7 in the "Leg 194 Summary" chapter). Toward the southeast, the facies of Megasequence C changes into a more irregular high-amplitude unit dipping toward the northwest. This indicates potential sediment shedding from the SMP and accumulation of platform-derived sediments in an apron at the base of the escarpment. At Site 1192, seismic Megasequence C/D boundary is calculated to occur at 240 mbsf.
Megasequence C correlates roughly with lithologic Unit II (late Miocene). The generally low amplitude seismic facies reflects a homogenous lithology over most of Megasequence C. This conclusion is supported by very monotonous mudstone lithology between 100 and 200 mbsf and by almost constant bulk density values (Fig. F30). The only high-amplitude couplet of reflections right below 750 ms TWT coincides with increases in gamma ray attenuation density at 200 and 230 mbsf (lower dashed line inn Fig. F30). Megasequence B/C boundary at 240 mbsf correlates approximately with the boundary between lithologic Units II and III (259 mbsf). This interval is characterized in the geochemical signature of the sediments by the lowest measured carbonate content of this site (65 wt%), by a negative excursion in the Hydrogen Index, and by a very high C/N ratio (see "Geochemistry"). All these sedimentological and geochemical "anomalies" point toward a relative increase of noncarbonate input into the depositional system during a period that can be seismically correlated with the exposure of the NMP. Using the established age model, this sequence boundary can be dated at ~10.5 Ma (see "Age Model").
The top of Megasequence B is characterized by a more or less conformable succession of low-amplitude reflections that increase in amplitude between 800 and 850 ms TWT. Hole 1192B was abandoned within these stronger reflections that dip to the southeast and terminate about 30 km updip toward the northwest in the more chaotic, partly horizontally stratified seismic facies of the NMP.
Cores at Site 1192 penetrated the upper part of Megasequence B, which incorporates lithologic Units III, IV, and V, all of middle Miocene age. The top of lithologic Unit III at 259 mbsf is characterized by abundant glauconitic grains and coincides fairly well with seismic Megasequence B/C boundary at 240 mbsf, using the proposed time-depth correlation. In addition to the geochemical signature of B/C described above, the boundary also marks the top of an ~50-m-thick interval that is rich in coarse glauconitic and phosphatic grains. Lithologic and petrophysical changes within lowermost Units IV and V correlate with high-amplitude reflections at the base of the hole.